Method of determining an angular offset of a position sensor

Abstract

A method of determining an offset between an angular position indicated by a rotary position sensor assembly secured directly or indirectly to the rotor of a multi-phase permanent magnet AC motor and an actual electrical position of the motor rotor, includes the steps of: applying a voltage vector to the motor, which is aligned with a known angular position of the motor; waiting for the motor rotor to move to a location in which the vector when considered in the dq frame of reference is centered on the d-axis; measuring the angular position of the rotor indicated by the position sensor, and determining the offset from the difference between the measured angular position and the known actual position of the vector.

The invention claimed is: 
     
       1. A method of determining an offset between an angular position indicated by a rotary position sensor assembly secured directly or indirectly to a motor rotor of a multi-phase permanent magnet AC motor and an actual electrical position of the motor rotor, the motor comprising a plurality of phases, the method comprising the steps of:
 applying a vector to the motor, which is aligned with a known angular position of the motor, and that alternates between a first value in which the vector is slightly advanced of the known position and a second position in which the vector is slightly behind the known position so as to cause the motor rotor to dither back and forth across the known position; 
 waiting for the motor rotor to move to a location in which the vector when considered in a dq frame of reference is centered on a d-axis; 
 measuring the angular position of the rotor indicated by the position sensor; 
 determining the offset from the difference between the measured angular position and the known actual position of the vector; and
 controlling an actuator comprising the motor based on the determined offset. 
 
 
     
     
       2. The method of  claim 1 , further comprising the step of selecting a vector which is aligned with a first one of the motor phases corresponding to zero degrees electrical of the motor. 
     
     
       3. The method of  claim 2  further comprising the step of applying a vector that is fixed in position relative to the motor stator during the measurement time and aligned with the one phase, so that current only flows in through one phase and flows equally out of the other phases of the motor. 
     
     
       4. The method of  claim 3  when used with a three phase motor with phases A, B and C, the method further comprising the steps of:
 applying values of A+B and A+C vector respectively for fixed alternate time intervals to cause the rotor to dither about an angular position centred on Phase A; 
 determining the angular centre position about which the rotor dithers and 
 determining the offset between that position and the actual position. 
 
     
     
       5. The method of  claim 4  wherein the centre position is derived by determining the average position of the rotor from the varying output of the angular position sensor. 
     
     
       6. The method of  claim 5  further comprising the steps of:
 at a later time applying a vector aligned with a second, different, known angular position on the motor stator; 
 waiting for the motor rotor to move to a location in which the vector when considered in the dq frame of reference is centered on the d-axis; 
 measuring the angular position of the rotor indicated by the position sensor; and 
 determining the offset from the difference between the measured angular position and the known actual centre position of the second vector. 
 
     
     
       7. The method of  claim 6  further comprising the step of:
 applying vectors that have known positions that change in a sequence that moves in a single direction around the stator over time such that the change of vector position causes the rotor to follow the vectors around the stator and step through a range of electrical positions that are increasingly offset both electrically and mechanically from the start position set by the first vector. 
 
     
     
       8. The method of  claim 7  further comprising the step of:
 repeatedly moving the vector and taking position measurements until the rotor has moved by one full mechanical revolution, and taking an average of the estimates to produce an overall estimate of the offset. 
 
     
     
       9. The method of  claim 8  further comprising the step of:
 for each estimate obtained from each fixed or dithered vector position, performing a check that an estimated value does not deviate from any other estimated value by an amount greater than a set threshold, and in the event that it does raising an error flag. 
 
     
     
       10. The method of  claim 9  further comprising the step of:
 generating a rotating αβ frame vector, before any estimate are made, that will cause the rotor to rotate through one continuous mechanical revolution and during the rotation measuring the angular position of the rotor, 
 whereby, in the event that the angular position does not indicate that the rotor has moved through one full mechanical revolution, the method determines that the motor rotor has hit an endstop, and, only in that event, subsequently generating a rotating αβ frame vector that will cause the rotor to rotate through an angle greater than or equal to one continuous mechanical revolution in the opposite direction and permitting the rotor to move to follow the vector. 
 
     
     
       11. A method according to  claim 10 , wherein the vectors are pulse width modulated voltages. 
     
     
       12. A linear actuator comprising:
 a motor having a rotor; 
 a mechanical arrangement which converts rotation of the rotor into a linear translation of an output part, the range of movement of the rotor being limited by the allowable range of movement of the linear part; 
 a rotary position sensor assembly fixed to the rotor; and 
 a signal processing unit arranged in use to determine an offset between an angular position indicated by the rotary position sensor assembly and an actual electrical position of the motor rotor by carrying out the steps of the method of any preceding claim.